1. Field of the Invention
The invention relates to a cassette for thermal transfer ribbon for setting images on printing plates, the cassette having spoons inside a housing for holding the thermal transfer ribbon.
2. Description of the Related Art
EP 0 698 488 B1 discloses a strip-like transfer foil for setting images on printing plates by means of a laser beam. The strip width of the transfer foil is small, as referred to the printing-plate width, and the transfer foil is spooled from a supply roll on to a rewind roll by means of a ribbon transport device and, in the process, led between printing plate and laser head. In order to reach the complete image area of the printing plate, the plate cylinder is rotated under control and the ribbon transport device, together with the laser head, is moved under control over the width of the plate cylinder on a transversing unit.
In order to carry out a thermal transfer process, the individual rolls of ribbon have to be inserted manually into the ribbon transport or guide device and the ribbon has to be threaded manually around the guide rollers and fixed to the rewind roll. In the process, rotation of the thermal transfer ribbon can easily occur, as a result of which the thermal transfer process does not function, since the functional layer is turned away from the surface of the printing plate.
DE 198 11 030 A1 describes a thermal transfer ribbon having a number of tracks running parallel to one another, which is made up ready in a cassette. After running through a complete track length, the ribbon is spooled back and an unused track is aimed at the laser, so that a further image-setting operation can be performed. After a cassette has been used up, a higher-order administration system with appropriate control means provides a further one.
In such cassettes friction occurs between the transfer ribbon and guide parts, so that only limited ribbon speeds are possible and the ribbon wears easily or the functional layer is damaged.
On this basis, the object of the invention is to provide a cassette of simple construction for thermal transfer ribbon which permits high ribbon speeds and rules out wear on the transfer ribbon.
According to the invention, this object is achieved by a cassette that comprises a housing and at least first and second spools carried rotatably in the housing so that a thermal transfer ribbon can be passed between the spools in a spooling operation.
The spools are carried in the housing having a play with respect to spool mounting in the housing so that the spools are positionable with respect to the housing such that neither the spools nor the transfer ribbon come in contact with any housing part during the spooling operation.
As a result of the particularly advantageous configuration of the cassette, the thermal transfer ribbon can be transported completely decoupled from the housing, that is to say neither the ribbon nor the spools touch the housing during spooling. As a result, friction and electrostatic charges are prevented, as a result of which the functional layer is subjected neither to wear nor damage, and the thermal transfer process can run without being influenced. In addition, because of this decoupling, the thermal transfer ribbon can be transported at very high speeds (for example 6 m/s).
The ribbon can advantageously be ready-made for various printing-plate sizes, is protected against damage in the cassette, and the individual wear-free parts, such as the housing and the spools, can be reused repeatedly with unused thermal transfer ribbon.
As a result of ready-making the thermal transfer ribbon in the cassette, rotation of the ribbon during threading is ruled out and, by means of a cassette designed in accordance with the invention, an automated threading operation for the transfer ribbon into an image-setting device is made possible.
It is particularly advantageous that the cassette does not have to perform any ribbon guiding tasks and the cassette can therefore be constructed particularly simply. Neither bearings for the spools nor ribbon guide rollers are required on the cassette, which makes particularly economic production possible. This aspect is also advantageously assisted by assembling the housing from two identical half-housings.
The cassette provides a transport safeguard which prevents inadvertent unwinding but permits rewinding in order to tension the ribbon. This advantageous safeguard is preferably an integral constituent part of the housing of the cassette and may be released both manually and also automatically.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.